Great story! I love the way you made it short and spare, with only the necessary details, but with a lot of humor and affection in it. And I love the tension of waiting to know what she would say.
I used to live in Newton, outside Boston, and I would daily bicycle out to Walden after work in the summer and go skinny dipping at night. I also went there in winter to go skating and I loved listening to the booming whiplash of the ice bowing under the weight of my friends and me. I even skinny dipped in April when ice still rimmed the edges of the lake.
You might have had a much easier and more romantic time coming into the park from the Lincoln side, from the direction going out towards Great Meadows (a place that is much closer to what Thoreau knew than present day Walden). It's much quieter, with almost no cars.